There is a desire in many fields to measure or recognise changes at an object such as in the distance to an object, for example, in order to derive control commands therefrom for any desired arrangements. Thus, a door opens when a measurement arrangement detects the approach of an object. Or a revolving door stops as soon as an obstacle is detected. Changes can occur as a result of the approach, the presence or removal of an object and are detected by the measuring device.
One possibility of distance measurement lies in the use of the light transit time. In this case, a laser beam is directed onto the object to be measured and the reflected light is measured. The delay until the reflected light arrives at the receiver again is a value for the distance that the light has covered. The speed of light amounts to about 300 000 km/s. A light pulse covering a path of 3 m needs approximately 10 nanoseconds to do so. High-speed lasers respectively high-speed photodiodes and amplifiers are necessary to determine the light transit time in a meaningful manner in the case of such short light paths.
One solution uses the possibility of transforming the transit time information into a frequency range that is easier to handle (cf. e.g. DE 100 22 054 A1). For this, to determine the transit time the emitted light is modulated with a high frequency, e.g. some hundreds of MHz. The received light is then mixed with a second frequency, which differs only slightly from the transmitted frequency. A third, significantly lower frequency that can be processed in a circuit more easily than the original high modulation frequency is formed as a mixed product. In this third frequency the information of the light transit time lies in the phase. Since the third frequency is generally mixed into a frequency range of some KHz, the determination of the phase information, and therefore the light transit time, is very simple. The determined difference value is used to actuate a phase shifter configured as a digital delay element and the delay time is changed until the difference value is low. The disadvantage of this system is that only a specific distance range is covered, outside of which the phase information is repeated periodically, so that uncertainties occur. Extensive measures such as the modulation with different frequencies, for example, are necessary to avoid these uncertainties. This system operates on principle with more than one light pulse, since otherwise no mixing process can occur. In known systems at least some thousands of individual pulses are emitted to obtain a plurality of periods of the signal mixed to the third frequency.
A second method of determining the light transit time is the direct measurement of an individual pulse. Uncertainties that are a disadvantage in the above-described method are excluded because of this. The advantage of an individual pulse is the possible higher power. However, significantly higher requirements are also set for the detection of such a light pulse in the receiver. To measure distances of less than 15 cm, the receiver must have a reaction time of less than a nanosecond. However, a perfectly emitted light pulse with a rise time of theoretically zero is “blurred” by the naturally limited bandwidth of the receiver. In an evaluation with a threshold value of 50% of the maximum amplitude, for example, a “delay” thus occurs that is generally dependent on temperature, but also on the received energy.
In practice, reflection differences of e.g. 1:30 000 occur between highly reflective surfaces, e.g. mirrors, and highly light-absorbent surfaces, e.g. black suede. To nevertheless generate a usable measured value, the received signal is generally regulated to a fixed amplitude. This regulation of the amplitude can lead to an undesirable time lag. The determination of the exact instant of receipt in the case of very small signals, and therefore a high noise component, has also proved difficult. Moreover, in the case of high reflection the received pulse should not overload the photodiode or preamplifier, since the instances of non-linearity occurring in that case have a negative effect on the accuracy of the receipt time.
Furthermore, a high-speed photodiode, generally an avalanche photodiode, is necessary for this method just as for the first-described method. Moreover, if the received signal is not already mixed with the second frequency in the photodiode, as in the first-described method, a high-speed preamplifier must also be provided here. Frequency ranges into the gigahertz range are no rarity. An additional factor is also a possible influence of extraneous light, which in an extreme case is some thousands of times stronger than the reflected light of the emitted pulse. All these influences have a negative effect on the accuracy of the measurement.
A light transit time measurement system is known from the later published patent application DE 10 2005 045 993.5, in which the light coming from continuously alternately clocked light paths is adjusted to a value of equal magnitude in the receiver by an amplitude regulation, and in this stabilised state the received signals are continuously checked for clock pulse change signals at the transitions between the clock pulses. These clock pulse change signals are then regulated to zero by shifting the phase preferably of both light paths in contrary direction to one another, wherein the adjusted phase shift corresponds to the light transit time to and from an object and thus to the distance to the object. With this solution the clock-synchronous signal component in the receiving path becomes zero with ideal switching behaviour of the light sources and correct phase shift control. This means that only amplifier noise is possibly present at the output of the photodiode amplifier. The method works exclusively with continuously consecutive pulses alternately coming from two different, but electrically identical light sources and is therefore restricted in its pulse power because of the temperature limit in the light-emitting diodes used. There is no electrical signal present that is integrated over at least one sub-section of the transmitted signal or reference signal resulting from these signals that are, moreover, close in time.
However, a disadvantage with this method is the possible occurrence of a signal that is not adjustable exactly to zero resulting, for example, from an asymmetry of the light sources or other electronic components used. This residual signal can lead to a measurement error in the case of high incident extraneous light radiation and at the same time high reflection of the transmitted signal. Said asymmetry occurs, for example, when an LED behaves differently with respect to time during switching on and off or when different light sources, e.g. lasers and LEDs, are used in combination.